The simultaneous elimination of the three major automotive pollutants ("3-way control") entails both chemically reducing reactions (for nitric oxide) and chemically oxidizing reactions (for carbon monoxide and hydrocarbons) in passage of the emissions stream over a single dual function catalyst, see U.S. Pats. Nos. 3,951,860, dated Apr. 20, 1976 and 3,898,181 dated Aug. 5, 1975. Alumina-supported catalysts containing rhodium in combination with other precious metals (typically platinum and/or palladium) have been commercially used as 3-way catalysts. Rhodium is effective for catalyzing the reduction reactions, while the other metals act to increase and stabilize the oxidation activity.
Our laboratory studies of fresh and thermally aged samples have shown that adding platinum and/or palladium to an alumina-supported rhodium catalyst, while increasing oxidation activity, causes more ammonia formation in rich mixtures and poorer over-all nitric oxide control in lean mixtures than is achieved over rhodium alone. This results from the fact that Pt and/or Pd, unlike Rh, act on NO to form primarily NH.sub.3 rather than N.sub.2 under reducing conditions. Under oxidizing conditions, though both NO and O.sub.2 are oxidizing agents for the CO and HC, the O.sub.2 most often reacts preferentially as the oxidizer and thus leaves the NO to pass through unreacted. This is especially true when Pt and/or Pd is present to catalyze reactions with O.sub.2. Further, though rhodium compares favorably with platinum and palladium as an oxidation ctalyst, total availability of rhodium and its low relative content in the ore as mined imposes practical limits on the loading on the support, and low loading of rhodium limits its oxidation performance.
In order to use 3-way catalysts, the art has developed a control system which is to operate the engine with the stoichiometrically correct air-fuel ratio to provide exhaust with just the proper balance of oxidizing agents (O.sub.2, NO) and reducing agents (CO, HC, H.sub.2). An ideal catalyst then could catalyze reactions to the entire mixture and lower all three pollutants simultaneously, see U.S. Pat. No. 3,696,618, dated Oct. 10, 1972. Though this system has the advantage of using a single catalyst bed, and stoichiometric engine operation gives better fuel economy than the rich operation required for the dual-bed approach, it does, however, require a precise feedback-controlled fuel metering system governed in part by an oxygen sensor in the exhaust stream. The continual sensing and readjustment of the air-fuel ratio causes the exhaust to fluctuate about the stoichiometric composition. This places additional demands upon the 3-way catalyst in that it must maintain its effectiveness on both the rich and lean sides of the stoichiometric point and is subject to the limitations in conversion noted above.
An alternative system disclosed by the art, see U.S. Pat. No. 3,953,576, dated Apr. 27, 1976, is to operate the engine fuel-rich to provide an oxygen-deficient (reducing) exhaust in which nitric oxide reduction (by CO, HC, and H.sub.2) can be catalyzed. Then air is pumped into the exhuast as it passes into a second catalyst bed for oxidizing the remaining CO and HC. This is obviously an expensive design and adds additional weight to the vehicle by reason of the additional pump and auxiliary control equipment required.